SON68 glass alteration under Si-rich solutions at low temperature (35–90 °C): kinetics, secondary phases and isotopic exchange studies

Abstract

Pristine and ²⁹Si-doped SON68 glass was leached in dynamic mode under silica-rich synthetic Callovo-Oxfordian (COx) groundwater at pH 8, a glass surface-to-liquid volume (S/V) ratio (900–1800 m⁻¹) and at 35, 50 and 90 °C. The solutions were analysed by ICP-MS and ion chromatography, the alteration products were studied by electron microscopy and Raman spectroscopy, and the Si-isotopes profiles were obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The glass alteration seems to be governed by both diffusion and surface reaction processes. After 653 days of alteration the normalized leaching rates were 2.6 (±0.2) × 10⁻⁵, 9.9 (±0.8) × 10⁻⁵ and 2.4 (±0.2) × 10⁻³ g m⁻² d⁻¹ at 35, 50 and 90 °C, respectively. The major alteration secondary phase is clay-type Mg-silicates at all temperatures in addition to powellite and apatite at 90 °C. SIMS studies clearly showed uptake of ²⁹Si by the surface gel via condensation and in the Mg-rich phyllosilicates via precipitation from solution. The precipitation of phyllosilicates at all temperatures constitutes the main process which destabilises the gel layer, thus maintaining a long-term glass dissolution rate in COx groundwater, higher than in pure water.